The present invention generally relates to a precursor and gas mixing unit, and more particularly relates to an apparatus for mixing vaporized precursor with a gas for producing pyrogenically generated silica particles such as those used in making fused silica and optical fiber preforms.
Conventional optical fiber preforms typically include silica particles. Halide free cyclo-siloxanes, such as octamethylcyclotetracyloxane (OMCTS), are typically used as a precursor for producing pyrogenically generated silica particles which are currently used in making fused silica and the optical fiber preforms. The precursor typically is a liquid that is vaporized in a vaporizer and flows in a stream through a delivery tube to combine downstream with oxygen in a mixing unit. The mixture of precursor and oxygen is then fed to a burner where the mixture undergoes oxidation reaction at high temperature in the presence of a flame to form silica particles. At the junction where the oxygen and the precursor are mixed, the precursor (which includes the presence of impurities such as high molecular weight siloxanes, non-volatile residue, amines, silanes, acids (e.g., HCl), and bases (NaOH, KOH)) can polymerize with oxygen resulting in the formation of a gel. Gelling in the delivery system generally results in increased back pressure and requires periodic cleaning of the equipment to remove the gel layer. This results in equipment down time and makes the manufacturing process inefficient. It is therefore desirable to provide for a precursor and gas delivery system that reduces or eliminates the undesirable gel formation.